Elevated CO2 effects on canopy and soil water flux parameters measured using a large chamber in crops grown with free-air CO2 enrichment.

An arable crop rotation (winter barley–sugar beet–winter wheat) was exposed to elevated atmospheric CO2 concentrations ([CO2]) using a FACE facility (Free-Air CO2 Enrichment) during two rotation periods. The atmospheric [CO2] of the treatment plots was elevated to 550 ppm during daylight hours (T > 5 °C). Canopy transpiration (EC) and conductance (GC) were measured at selected intervals (>10% of total growing season) using a dynamic CO2/H2O chamber measuring system. Plant available soil water content (gravimetry and TDR probes) and canopy microclimate conditions were recorded in parallel. Averaged across both growing seasons, elevated [CO2] reduced EC by 9%, 18% and 12%, and GC by 9%, 17% and 12% in barley, sugar beet and wheat, respectively. Both global radiation (Rg) and vapour pressure deficit (VPD) were the main driving forces of EC, whereas GC was mostly related to Rg. The responses of EC and especially GC to [CO2] enrichment were insensitive to weather conditions and leaf area index. However, differences in LAI between plots counteracted the [CO2] impact on EC and thus, at least in part, explained the variability of seasonal [CO2] responses between crops and years. As a consequence of lower transpirational canopy water loss, [CO2] enrichment increased plant available soil water content in the course of the season by ca. 15 mm. This was true for all crops and years. Lower transpirational cooling due to a [CO2]-induced reduction of EC increased canopy surface and air temperature by up to 2 °C and 0.5 °C, respectively. This is the first study to address effects of FACE on both water fluxes at canopy scale and water status of a European crop rotation.